Fun and simple, and oddly meditative once I got into the groove! As others have said already, it was confusing at first that the squares don't start gray and have to be cycled through, though the additional time pressure arguably made it more fun once I figured it out.
Also, as a "javascript" "dev", I'm impressed you wrote an entire, nicely polished game in 48 hours in javascript! Seems like a perfect fit for this game :)